Hard-wired security systems in which danger sensors are connected to a central unit via a two-wire line or a bus (see for example WO-A-95/24087), are well known. Wireless security systems are also known and are becoming increasingly common (see e.g. U.S. Pat. No. 5,155,469). Hybrid systems in which both types of communication, wired and wireless, are used (see for example EP-A-O 803 850), are also known.
A hard-wired connection guarantees a very high reliability of transfer, but makes the entire system relatively inflexible when seeking to accommodate changes in room designs and use. Such changes usually require the relocation of individual sensors which because of the necessary wiring can require extensive, disruptive and expensive installation work. The subsequent fitting of such security systems with wires in existing buildings, particularly in those in which no security system was originally provided, can be complex and expensive. This is particularly so in historic buildings.
Wireless security systems which have been used in private houses are known to be susceptible to faults, one attributing factor of which is the systems inadequate battery energy supply. This disadvantage however has been largely eliminated now that long-life batteries are available, and further because the power consumption of the radio sensors has been able to be reduced. Other causes of fault such as for example false alarms may also be reduced by alarm verification measures.